"to inflame, to burn," late 15c., from Latin combustus, past participle of combuere "to burn up, consume" (see combustion). "Now only jocular or affected" [OED]. Related: Combusted; combusting. Combust was used in Middle English from late 14c. as a past-participle adjective, "burnt," from Old French combust (14c.) and directly from Latin combustus. Also it was an astrological term for planets when near the sun.